Ypres occupied a strategic position during the First World War because it stood in the path of Germany’s planned sweep across the rest of Belgium and into France from the north (the Schlieffen Plan). The neutrality of Belgium was guaranteed by Britain; Germany’s invasion of Belgium brought the British Empire into the war. The German army surrounded the city on three sides, bombarding it throughout much of the war. To counterattack, British, French, and allied forces made costly advances from the Ypres Salient into the German lines on the surrounding hills.

In the First Battle of Ypres (19 October to 22 November 1914), the Allies captured the town from the Germans. The Germans had used tear gas at the Battle of Bolimov on 3 January 1915. Their use of poison gas for the first time on 22 April 1915 marked the beginning of the Second Battle of Ypres, which continued until 25 May 1915. They captured high ground east of the town. The first gas attack occurred against Canadian, British, and French soldiers, including both metropolitan French soldiers as well as Senegalese and Algerian tirailleurs (light infantry) from French Africa. The gas used was chlorine. Mustard gas, also called Yperite from the name of this town, was also used for the first time near Ypres, in the autumn of 1917.